PM: docs: Fix stale reference in kernel-parameters.txt

Commit 7aa7a036 (PM: docs: Delete the obsolete states.txt
document) forgot to update kernel-parameters.txt with a reference
to the new sleep-states.rst document and it still points to
states.txt that was dropped, so fix it now.

Fixes: 7aa7a036 (PM: docs: Delete the obsolete states.txt document)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>